Carelessness on top

The most common cause of Arc Flash and other electrical accidents is carelessness. No matter how well a person may be trained, distractions, weariness, pressure to restore power, or overconfidence can cause an electrical worker to bypass safety procedures, work unprotected, drop a tool or make contact between energized conductors.

10 Common Causes Of Arc Flash and Other Electrical Accidents (photo credit: cusi.biz)

Faulty electrical equipment can also produce a hazard while being operated. Electrical safety hazards such as exposure to shock and Arc-Flash can be caused by:

Carelessness

Worn or brok­en conductor insulation

Exposed live parts

Loose wire connections

Improperly maintained switches and circuit break­ers

Obstructed disconnect panels

Water or liquid near electrical equipment

High voltage cables

Static electricity

Damaged tools and equipment

The severity and causes of electrical hazards are varied, but the best protection is to deenergize equipment before working on it.

No one has ever been killed or injured from an Arc-Flash while working on deenergized equipment. If equipment cannot be deenergized, electrical workers must be “qualified”, trained, wear appropriate personal protective equipment (PPE), and follow all applicable OSHA and NFPA standards.

Both OSHA and NFPA 70E require an Electrical Hazard Analysis prior to beginning work on or near electrical conductors that are or may become energized.

The analysis must include all electrical hazards:

Shock,

Arc-Flash,

Arc-Blast, and

Burns.

NFPA 70E Article 110.8(B)(1) specifically requires Electrical Hazard Analysis within all areas of the electrical system that operate at 50 volts or greater. The results of the Electrical Hazard Analysis will determine the work practices, protection boundaries, personal protective equipment, and other procedures required to protect employees from Arc-Flash or contact with energized conductors.

What determines the severity of an Arc Flash?

Several groups and organizations have developed formulas to determine the incident energy available at various working distances from an Arc-Flash.

In all cases, the severity of the Arc-Flash depends on one or more of the following criteria:

Available short circuit current

System voltage

Arc gap

Distance from the arc

Opening time of overcurrent protective device (OCPD)

Example of Slow Motion Arc Flash 100A Disconnect showing the impact of body position in arc flash. In the video, the door comes off in less than 2 cycles showing that Personal Protective Equipment (PPE) isn’t the only thing to consider when it comes to arc flash. Body position at the point of contact is also important.

When a severe enough Arc-Flash occurs, the overcurrent protective device (fuse or circuit breaker) upstream of the fault interrupts the current.

The amount of incident energy a worker may be exposed to during an Arc-Flash is directly proportional to the total clearing ampere-squared seconds (I2t) of the overcurrent protective device during the fault.

High current and longer exposure time produces greater incident energy. The only variable that can be positively and effectively controlled is the time it takes for the overcurrent protective device to extinguish the arc. A practical and significant way to reduce the duration of an Arc-Flash and thereby the incident energy is to use the most current-limiting OCPD’s throughout the electrical system.

Arc Blast Effect

During an Arc-Flash, the rapidly expanding gases and heated air may cause blasts, pressure waves, or explosions rivaling that of TNT. The gases expelled from the blast also carry the products of the arc with them including droplets of molten metal similar to buckshot.

For example, the high temperatures will vaporize copper, which expands at the rate of 67,000 times its mass when it changes from solid to vapor. Even large objects such as switchboard doors, bus bars, or other components can be propelled several feet at extremely high velocities.

In some cases, bus bars have been expelled from switchboard enclosures entirely through walls (can you believe it??).

Blast pressures may exceed 2000 pounds per square foot, knocking workers off ladders or collapsing workers’ lungs. These events occur very rapidly with speeds exceeding 700 miles per hour making it impossible for a worker to get out of the way.

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Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry facilities. Professional in AutoCAD programming.

16 Comments

SURESH DUGGAL

Sep 11, 2019

Dear Advard,
I am a technical expert on Circuit Breakers working as Technical Advisor with India’s leading electrical firm HAVELLS INDIA LTD.
In my opinion if there is a localized heating continuously on busbar joint may be due to poor connection or less cross section or low conductivity etc. slowly slowly start reducing the insulation level between phases because mostly GPO3 grade material is used and B class insulation after 130 degree centigrade & above start losing insulation level ultimately cause sparking between phases.
Once stared its ionized particles spread on other phases also results in flashover.
Again if there is a draw-out type circuit breaker, heating may cause the failure of MIC (Power contacts) because spring steel loose its tension and reduction in contact pressure cause flashover.

Knowledge is power. There are too many workers unaware of danger of working on live electrical equipment.
The more I get educated about available arc flash levels at various voltage levels, the more I try to get protected and the more I am thinking about a job to be performed.
Still, the electricians working for small size companies have no idea what danger is lurking inside the electrical enclosures; be it a small 120V or 600V breaker panel or a 600V switchgear. Far too many times, I have witnessed electricians working on live panels with just a short sleeve shirt or a T-shit. The fact is, that a small 600V electrical panel with only 4 breakers can pose real danger with incident energy to kill a worker.
How could it be, that this small panel is more dangerous, than the much larger panel located just next to it? The main factor that influences the available arc flash level is the upstream source and the upstream protection. Also, how correctly the co-ordination study was performed, how the protection devices were setup.
We used to open a main secondary transformer box for infrared survey. A 4,000 kVA transformer with 600V secondary. Knowing what I know today, I would never do it again. The same applies to the main breaker cubicle in a 600V switchboard. No more infrared surveys unless the switchboard is equipped with infrared viewing ports. Or can be powered down for an inspection.
Statistically, the most dangerous task is racking-in a breaker into a live cubicle. How to limit a danger? Power down the cubicle.
While commissioning large electrical systems, I came across few breakers that did not fit the buss bar correctly. If I did not spot the defect, this would became a potential future disaster.
Today, the co-ordination studies are more focused on limiting danger to workers than to limit the nuance tripping. That follows a recent US court decision that sided with an injured worker who by his own omissions caused a short in an electrical panel and was injured. An electrical engineering company was found guilty by setting up the protection devices in order to limit the nuance tripping rather than to protect a worker working on the electrical panel.
The fact is, the lower voltage is not always safer. The arc flash damage depends on the available fault levels.
The arc flash labels are the proper way to establish the use of personal protective equipment (PPE).
I am an experienced person and I can assess the possible and obvious fault levels just by looking around: checking transformer sizes and upstream breaker settings. Unfortunately, there are many old electrical installations that do not have arc flash labels. And there are too many workers unaware of danger of working on live electrical equipment.
Knowledge is power to be protected.

I wonder that, with the actual technologies of MV switchboards (arc-proof) and the safety requirements of EN standards to operate swithgears’ equipments, so much time is beeing loose to discuss arc flash.
Ask the users that have installed RMU and other arc-proof switchgears, and control the temperature and the quality of the air in switchgears’s rooms if they have problems with arc flash.
You are in 21sts century, not in the fifties of 20th century.
To be an expert on LV high power busbar trunking and LV/MV switchgears. (????) is not enough to discuss such kind of situations.
I think that Mr.Edvard Csanyi talks to much about toppics he is not na expert.
And, by the way, I’m prepared to be censored.

Anyway, I disagree that arc-flash incidents in MV switchgears are not very often, at least not in my 15y expirience. In the matter of fact, in that time I heard of 4-5 incidents with 6-7 dead engineers in total. And MV switchgears are average old about 7-12 years.

Caution is a must, as well as a complete understanding of what creates arc-flash and how to protect yourself. This article is about it. Don’t underestimate the possibility of it. All major manufacturers pay a huge attention to safety when working in substation with energized or de-energized equipment.

By the way, I do a dosen of other things other than LV busbar design, but it would be a kind of stupid to write essays of it, wouldn’t it?

It cannot be over emphasis, the importance of what has been stated above. High voltage operation is a serious business that involved ones life. The employee needs to know a lot about how to protect himself and what things that can harm him/her. Hope, that some gear switch designs are suicidal and out dated but are still in use is not helping matters. When it comes to safety. All designs should as a matter of safety incorporate all fixtures that will ensure the safety of personnel.

Not just that the employee needs to know certain things, but that some gear switch designs are suicidal and out dated but are still in use. All designs should as a matter of safety incorporate all fixtures that will ensure the safety of personnel.